The invention relates to a method for recording information in the form of magnetic domains in a material, a recording device for carrying out the method and an antenna structure for use in the recording device.
Conventional magnetic recording, such as hard-disk recording, uses a magnetic field to write magnetic domains in a material, the domains representing information recorded. The material used in this type of magnetic recording is usually a sputtered ferromagnetic film and has typically a granular structure. The remanent magnetization is typically high. The domains are being read using a magnetic head. In general, the size of the grains is of the order of (10 nm)^3 and the thickness of the recording layer in which the grains are embedded is also of the order of some tens of nm or less. The exchange coupling over the boundaries of the grains is small. The preferred magnetization orientation is usually longitudinal.
A different type of magnetic recording is magneto-optical recording, in which domains are recorded using a light beam to heat the recording layer and a magnetic field small compared to the write fields used in conventional hard disk recording to orient the magnetisation of the recording layer at the increased temperature close to the Curie temperature where the coercivity is sufficiently low to allow a magnetization reversal in the relatively small write field and are read using an incident light beam whose reflected beam is modulated by the perpendicular magnetization due to the Kerr effect. The magnetization is typically low compared to the magnetization common in hard-disk recording. Magneto-optical recording does not use granular material but a magnetically-continuous amorphous material in which there is a long-range exchange coupling between the spins of the material, the range extending well over a 100 nm. The preferred magnetization orientation is usually perpendicular. Activation volumes in MO media take over the role of the grains in granular media. Magneto-optical media are typically made of TbFeCo or GdFeCo ferrimagnetic layers, optimised to the Kerr effect and not to a strong remanence.
For high density media in conventional magnetic recording, small grains are necessary to avoid excessive medium noise. However, the magnetization state of grains considerably smaller than (10 nm)^3 is not stable over a sufficiently-long period. The instability can be avoided by increasing the coercivity of the media. For a strong increase of the density, the coercivity must be made so high that write fields need to be higher than can practically be realized. In thermally-assisted recording, also called heat-assisted magnetic recording (HAMR), the necessity of a high write field in a very-high coercivity medium is circumvented by increasing the temperature temporarily and thereby reducing the coercivity strongly at the location where the write field is active and the recording is taking place. It is noted that the coercivity of most materials decreases with increasing temperature. When the recording material has cooled down to room temperature after the recording process, the coercivity will be high again, providing a long-term stability of the written domain. This so-called thermally assisted recording is described in the article “Limits of conventional and thermally-assisted recording” by Jaap J. M. Ruigrok, J. Magn. Soc. Japan, 25, pp. 313–321 (2001). For very-high recording speed, i.e. magnetic field switching times well below 1 ns, another limiting factor arises, which is almost independent of the static (low speed) value of the coercivity. This limit is stated as follows, see the above cited article: the product of the time required for switching the magnetisation of a domain and the applied magnetic field cannot exceed a certain constant value, which value depends on the gyromagnetic ratio, which in turn is (almost) a physical constant. Hence, when decreasing the duration of the magnetic pulse, the magnetic field strength applied to the magnetic material during the pulse must be increased. For a field pulse far below 1 ns, the write field must be well above 800 kA/m (1 Tesla) and becomes unrealistic in practice, see e.g. the referenced article. It is explained in this article that the speed limitation also limits the improvement of the information density in thermally-assisted recording.
The disadvantage of the known thermally-assisted and conventional hard-disk recording methods is the relatively long minimum duration of the magnetic field pulse, the relatively high magnetic fields required and the limited density. It is the object of the invention to provide, amongst others, a method for recording at higher speeds and lower fields and a recording device using this method.
The above and following citations are hereby incorporated in whole by reference.